Electric coupling circuits



NOV. 10, 1931. WHEELER 1,831,431

ELECTRIC COUPLING CIRCUITS Filed Jan. 16, 1931 INVENTOR HAROLD A.WHEZER BY Q 9 Lu: J- M ATTORNEYS Patented 10, E931 UNITE PATET OFFICE' HAROLD A. WHEELER, OF GREAT NECK, NEW YORK, ASSIGNOR TO EAZELTINE CORPORA- TION, A CORPORATION OF DELAWARE nnnc'rnrc COUPLING crnou'rrs Application filed January 16, 1981. Serial No. 509,115.

This invention relates to high frequency electric coupling circuits-tunable over a range in frequency and adapted more especially for interconnecting successive elements of a high frequency thermionic amplifier or a radio system.

The coupling circuits of the present invention are characterized in that the voltage ratio produced thereby is caused to vary automatically with the frequency of tuning in a manner which is under the control ofthe designer; the design in general being pref erably such as to produce a substantially constant resultant amplification, i. e., uniform sensitivity throughout the tunable frequency range. 4

The subject matter of this invention has been published in a paper entitled Theory and operation of tuned radio frequency coupling systems by H. A. Wheeler and W. A. MacDonald. This paper was presented and distributed in printed form at the 1930 Convention of the Institute of Radio Engineers held at Toronto, Canada. f

With an elementary type of high frequency coupling circuit of which. a two-winding transformerhaving a tunable secondary is a typical example, the variation of amplification with frequency is not under the control of the designer, but increases with frequency throughout the tunable range in a manner well understood, with consequent detrimental effects on the amplification, selectivity and stability of the system.

In the elementary type of coupling circuit referred to above there exists between the primary and tunable secondary circuits :1. co-

above noted objections by proper coopera tion of different reactance elements in the primary circuit, These provide an effective coupling between input and tunable output r circuits which automatically decreases with proportioned to offset wholly or iii part the increase in the tuned frequency, and may be.

factors. operative to produce an increase in am lification with frequency.

oupling circuits broadly operative in accordance with the principles disclosed herein but difi'ering fr n the present invention as regards both the ircuit connection utilized and the specific modes of operation thereof,

are set forth in Patent No. 1,7 63,380 issued to C. E. Trube, and in his copending application Serial No. 101,906, filed April 14, 1926, and Serial No. 120,045, filed July2, 1926.

'The coupling circults of the present invention embody the following essential elements,

namely, a fixed inductance and the secondary coil of a high frequency transformer serially connected between an output terminal and a common or grounded terminal, a variable tuning condenser likewise connected betwee the output and grounded terminals, a pnmary transformer coil inductively related to the secondary connected between an input terminal and the common or a grounded terminal, and a fixed coupling condenser connected fromthe ungrounded terminal of the primary coil to a point in secondary circuit of opposite alternating current polarity thereto.

The mentioned connection of elements pro-. vides a pair of parallel paths between the input and grounded terminals; one path containing the fixed capacity and at least a por- Thus the coupling circuit of the present inv vention comprises. an input circuit coupled both capacitively and magnetically to a tunable secondary circuit. These couplings may be proportioned to provide a resultant coupling between primary and secondary circuits which automatically falls off as the frequency intuning is increased but at a rate proportioned to producea desired variation in amplification and selectivity with freqhency; and also to control the variation in input impedance of the circuit in such manner as to-insure stability of operation when utilized in conjunction with thermionicelements. a

In order to render the resultant coupling of maximum eflect at the low frequency end of the tunable range, the inductance of the primary coil is by association of suitable ca pacities including that of the coupling condenser, rendered resonant at a frequency lower, but not greatly lower, than the lowest frequency within the tunable range. To this end an additional fixed capacity may be connected between the input and ground terminals, or in the operation of the circuit capacity may exist between these terminals sufficient in magnitude to effect the desired low resonance frequency of the primary circuit. Such capaciiy may for example be present in the form of the antenna-to-ground capacity of an antenna circuit connected to the input and ground terminals of the coupling circuit, or in the form of the anode-toground capacity of a thermionic element connected thereto.

Referring now to the drawings:

Figs. 1, 2 and 3 show diagrammatically elementary coupling circuits in accordance with the present invention;

Fig. 4 shows the coupling circuit of Fig. 3 in association with an antenna circuit connected to the input portion thereof; while Fig. 5 discloses the manner of connecting the anode circuit of a thermionic element to the input portion of the Fig. 3 modification.

Like elements are similarly designated in several figures.

The coupling circuit of Fig. 1 comprises a variable tuning condenser C shunted by the series connection consisting of a fixed inductance L and a secondary transformer coil L bridged between output terminals 0, the lower terminal of which is conductively connected to the lower of a pair of input terminals I in order to provide a common or ground connection. Inductively related to the secondary coil L is a primary coil L connected between input terminals I. A fixed coupling condenser C extends between the ungronnded terminals of coils L and L respectively. Coil L is so poled relative to coil L. that the coupling condenser C is connected between points of opposite alternating current polarity in order that. with the circuit elements properly proportioned as explained, the capacitive coupling due to element will aid the magnetic coupling existing' between coils L and L throughout the tunable frequency range.

The capacity C shown dotted, is intended to represent the entire capacity effectively bridged between the input terminals I while the circuit is in operation, and in conjunc tion with capacity C and any additional capacities which may be associated with the primary coil L should be such as to render the primary circuit resonant at the mentioned frequency slightly below the tunable range of the secondary circuit. It is not essential to the operation of the circuit that minals I. In the event that capacity C is zero however, capacity C should be of such magnitude as to insure the desired low frequency resonance condition of the primary circuit.

The capacity C is sufficiently small that the path between the input terminals I which contains elements C and L is capacitively reactive throughout the tunable range and hence of decreasing impedance for an increase in the tuned frequency; whereas reactance of element L must increase in magnitude with the tuned frequency. It will thus be apparent thatelements C and L produce effects in the secondary circuit which vary in o posite fashion with the tuning adjustment, the resultant effect being proportioned to produce a resultant coupling between primary and secondary circuit which auton'iatically falls off in a desired rate as the frequency of tuning increases.

Since the instantaneous current flowing in the coupling condenser C is in phase oppositior to that flowing in coil L it is necessary in order that the capacitive aid the magneic coupling, to connect the condenser C between points in the primary and secondary circuits which are of opposite alternating current polarity.

The circuit of Fig. 2 constitutes a slight modification over that of Fig. 1 in that the coupling condenser C is connected between the ungrounded terminal of the primary coil L and the undgrounded terminal of the secondary tuning condenser C. This modification requires different circuit constants to obtain the same results as are obtained by the circuit of Fig. 1, but in other respects the two modifications are practically equivalent.

The circuit of 3 which is electrically equivalent to Fig. 2 differs therefrom in the connection of the fixed inductance L between the lower terminal of the secondary coil L and the condenser C rather than between the upper terminal of the secondary coil and the condenser as is the case in Fig. 2.

In Fig. 4 an antenna circuit A is connected to the input terminals I of the Fig. 3 modi fication. The capacity C in this instance comprises the antenna-to-ground capacity effectively bridged between the input terminals. and in conjunction with capacity C should be such as to render the primary circuit resonant at the mentioned frequency below the tuning range.

In Fig. 5 the upper-input terminal of the circuit of Fig. 3, is-connected directly to the anode of a thermionic element V whereas the lower input terminal is connected to the +13 lead extending to a space-current supply source for supplying the anode circuit of the tube through coil L Tube V is of the screen grid type wherein the screen grid electrode is connected to a denser.

suitable source of biasing potential over lead S. A grid biasing resistor R shunted by a high frequency by-passing condenser B is connected between cathode and ground for applying a suitable negative potential to the grid electrode of the tube in accordance with the usual present day practice.

The lower input terminal is not in this instance connected directly to -the,lower output terminal as this would result in grounding the +18 terminal of the space-current supply source at the grounded rotor element of condenser CI Instead the connection between these terminals is traced through the by-pass condensers to ground and thence directly to the grounded output terminal.

The capacity;. C comprises the anode-toground capacity of tube V which in association with capacity C provides the desired resonance condition for the primary circuit.

While the circuits of Figs. 4 and 5 depict the modification of Fig. 3 connected to an antenna circuit and to a thermionic element respectively. it will be apparent thatthe modificationsof Figs. 1 and 2 could be effectively employed in the same manner.

All of the circuit modifications are of such nature as to permit a direct grounding of the rotor element of the variable tuning con- This isdesirable in that it prevents undesired couplings otheiwise occurring between the rotor element and different partsof the circuit.

In this invention, especially in Figs. 3, 4 and 5, the capacity C can easily besecured as inherent capacity between coils L and L \What is claimed is: i v

1. A high frequency electric cou ling circuit tunable throughout a range in requency comprising a secondary circuit including a variable condenser bridged'between an output and a common ground terminal, a fixed inductance in series with a secondary transformer coil connected in shunt to sa d condenser, a primary transformer coil inductively related to said secondary connected between said ground terminal and an input terminal, and a fixed capacity connecting the ungrounded end of said primary coil to a point in said secondary circuit ofopposite open-circuit alternating current polarity, the inductance of said primary coil together with the associated capacities in the operation of said circuitbeing resonant at a frequency slightly below said tunable range.

2. A high frequency electric coupling circuit tunable throughout a range in frequency comprising a secondary circuit containing a variable condenser bridged between an out put and a common terminal, a fixed inductance in series with a secondary transformer coil connected in shunt to said condenser, a primer transformer coil inductively related to. sai secondary connected between said ground terminal and an input terminal, and

I the said capacities associated therewith being such as to render the primary circuit resonant at a frequency slightly below the tunable range for providing uniformly high sensitivity and selectivity throughout said tunable;

range. I

3. In a tuned radio frequency receiving system the combination of an electric couplingcircuit having an input, an output and a ground terminah'a variable tuning condenser shunted by a fixed inductance and a secondary transformer coil in series connected between said output and ground terminals, a primary coil inductively related to said secondary coil connected between said input and ground terminals, a fixed capacity connected from the ungrounded terminal of said primary coil to a point in said secondary circuit of opposite open-circuit alternating current polar1ty, and an antenna circuit associated with said input and ground termin'als, the inductance of said primary coil together with the associated capacities including said fixed capacity and the antenna-toground capacity of said antenna circuit being resonant at a frequency slightly below the tuning range of said variable condenserfor effecting in said system uniformly high sensitivity and selectivity throughout said range.

4. In a high frequency thermionic amplifying system, the combination of an electric coupling circuit having an input, an output and a ground terminal, a variable tuning condenser connected between said ontput and ground terminal and shunted by a fixed inductance and a secondary transformer coil in series, a primary coil connected between saidinput and ground terminals and inductively related to said secondary coil, a fixed capacity connected from the ungrounded terminal of said primary coil to a point in said secondary p) circuit of opposite open-circuit alternating current polarity, and a thermionic tube having anode and cathode connected to said input and groui-id terminals respectively, the inductance of said primary coil together with the associated capacity including said fixed capacity and the anode-to-groundcapacity of said thermionic tube being resonant at a frequency slightly below the tuning range of said variable condenser for effecting in said system a uniformly high sensitivity, selecfrequency range. In testimony. whereof I afiix myslgnature.

HAROI D A. WHEELER.

tlvity and stability throughout said tunable 

